This invention relates generally to disk drive systems. More particularly, this invention relates to a method and apparatus for searching to a target track in a disk drive.
Personal computers typically connect to an optical disk drive such as a CD-ROM to read data from a compact disk. On the compact disk, data is stored in the form of pits and lands patterned in a radial track. The track is formed in one spiral line extending from the inner radius of the disk to the outer edge. A pit is a location on the disk where data has been recorded by creating a depression in the surface of the disk with respect to the lands. The lands are the areas between the pits in the tangential direction. The reflectivity of the pits is less than the reflectivity of the lands. To store audio or digital information, the length of the pits and lands are controlled according to a predefined encoding format.
When reading information from the disc, light from a laser beam is directed onto the track and the light beam is reflected back to a photo-sensor. Since the pits and lands have different reflectivity, the amount of reflected light changes at the transitions between the pits and the lands. In other words, the encoded pattern of the pits and lands modulates the reflected light beam. The photo-sensor receives the reflected light beam, and outputs a modulated signal, typically referred to as an RF signal, that is proportional to the energy of the light in the reflected light beam.
In FIG. 1, the relationship of the RF signal to the pits and lands is shown. A smaller pit or land decreases both the period and the amplitude of the RF signal. The RF signal in the pits and lands has opposite polarity.
One encoding format used in optical disk systems is eight-to-fourteen modulation (EFM). EFM reduces errors by minimizing the number of zero-to-one and one-to-zero transitions. In other words, small pits are avoided. In EFM, the data signal includes no less than two zeros and no more than ten zeros between logical transitions at the pit edges. A zero is indicated by no change in the energy reflected beam for at least two clock periods. A one is indicated by a change in the energy of the reflected light beam, that is, a pit edge. Applying the EFM encoding rules, a pit or land will have a length corresponding to the amount of time for at least two and up to ten clock periods and the electronics will output a corresponding voltage as shown in FIG. 1.
In an optical disk drive, an optical head assembly includes the photo-sensor, a tracking actuator and a lens. The optical head assembly is mounted on a sled. The lens is not directly attached to the sled. The lens is mounted on the tracking actuator which is mounted on the sled. The lens is positioned between the photo-sensor and the disk to transmit the light beam from the photo-sensor onto the disk surface and to transmit the reflected light beam to the photo-sensor. The sled and tracking actuator position the lens with respect to the spiral track. The sled is driven by a sled motor that positions the optical head assembly radially across the disk. The tracking actuator is a voice coil motor (VCM) that positions the lens within the limits of the sled. Because the geometry of the photo-sensor is large with respect to a single track, the lens can be positioned within a range of tracks and the photo-sensor can properly detect the RF signal.
A search is performed to position the optical head assembly over a target region of the spiral track. During searching, even though the track is a spiral track, track crossings will be detected as the optical head assembly is moved radially across the spiral track. The track crossings provide relative position information with respect to an initial position on the disk.
For rough searches, the sled and sled motor provide primary positioning of the optical head assembly. For fine searches, the tracking actuator provides primary positioning of the lens in the optical head assembly. An actuator drive signal is used to control the tracking actuator. In some disk drives, a sled motor drive signal is generated by extracting low frequency motion information from the tracking actuator drive signal so that the sled follows the motion of the tracking actuator. This technique works well in normal play mode when the radial motion of the tracking actuator with respect to the spiral track is very slow.
During both play mode and fine search operations, some drives derive the sled motor drive signal from the actuator drive signal, while other drives derive the sled motor drive signal based on the number of tracks to jump. Neither of these methods use track position information. However, these techniques may cause a head crash during a fine search operation because the motion from the tracking actuator is not synchronized to the motion of the sled.
Mechanically, the sled is massive as compared to the tracking actuator. Therefore, the motion of the sled is comparatively slow and has a low frequency response as compared to the tracking actuator. During play mode, the tracking actuator continuously follows the spiral track and the radial motion of the tracking actuator is slow. Therefore, the sled will follow the tracking actuator without many problems.
During fine searches, some disk drives use the track crossing information to control the sled, but the resolution of the track crossing information is too high and too fast for the sled to follow. The track crossing information represents relative movement and not an absolute position. Because of the slow response time of the sled to the track crossing information, the sled may not respond to all track crossings and the position of the sled may become unknown. Therefore, the motion of the sled and the motion of the tracking actuator may become unsynchronized resulting in a head crash.
Some other disk drives derive a sled motor drive signal from the number of tracks to move the head. However, this technique is an estimate of the required sled drive signal that varies with the length of the search and the mechanical characteristics of the disk drive. In addition, this method requires a large gain table based on the number of tracks to move. A long play mode follows each search to correct for any errors in positioning the sled and the tracking actuator, which increases the access time.
In view of the foregoing, it would be highly desirable to provide a method and apparatus to synchronize the tracking actuator and sled during a fine search operation. Such a method and apparatus would provide a more resiliant disk drive system by reducing the potential for head crashes. In addition, by synchronizing the motion of the sled and the tracking actuator, the access time for search operations will be reduced.
A center error signal is used to control the sled during fine search operations. The center error signal is used as a feedback signal to synchronize the motion of the tracking actuator and the sled.
A head that is mounted on a sled in a disk drive is positioned by the method of the invention. The sled is positioned by a sled motor and a lens is positionally mounted on the sled. The lens is positioned with respect to the sled by a tracking actuator. A center error signal indicates a position of the lens with respect to a track on a disk. A tracks-to-jump signal specifies a predetermined number of tracks on the disk that the sled motor is to move. A modified tracks-to-jump signal is produced in response to the center error signal. The sled motor is controlled with the modified tracks-to-jump signal.
In another aspect of the invention, a circuit includes a center error signal generator to generate a center error signal that indicates a position of the lens with respect to a track on a disk. A sled motor signal generator receives a tracks-to-jump signal that specifies a predetermined number of tracks on the disk that the sled motor is to move, produces a modified tracks-to-jump signal in response to the center error signal, and controls the sled motor with the modified tracks-to-jump signal.
In yet another aspect of the invention, a disk drive uses the circuit of the present invention.
According to the principles of the invention the center error signal maintains synchronism between the tracking actuator and sled during the fine search operation. Therefore, the resilience of the disk drive is increased by reducing the potential for head crashes. In addition, by synchronizing the motion of the sled and the tracking actuator, the access time for search operations is reduced.